Power transfer systems of the type used in motor vehicles such as, for example, four-wheel drive transfer cases, an all-wheel drive power take-off units (PTU) and axle drive modules are commonly equipped with a torque transfer mechanism. Torque transfer mechanisms are operable to regulate the transfer of drive torque from a rotary input component to a rotary output component. Typically, a friction clutch is operably disposed between the input and output components. Engagement of the friction clutch is varied to regulate the amount of drive torque transferred from the input component to the output component.
The degree of clutch engagement is a function of the clutch engagement force that is applied to the friction clutch via a clutch actuator system. Traditional clutch actuator systems include a drive mechanism and a clutch operator mechanism. The clutch operator mechanism converts the force or torque generated by the drive mechanism into the clutch engagement force which is then applied to the friction clutch. The drive mechanism can be passively-actuated or, in the alternative, be a power-driven device which is controlled based on a control signal generated by a control system.
The quality and accuracy of drive torque transfer across the friction clutch is largely based on the frictional interface between the interleaved clutch plates. When partially engaged, the clutch plates slip relative to one another, thereby generating heat. As is known, lubricating fluid is routed to flow through the clutch pack to cool the clutch plates. In a typical clutch engagement cycle, the heat generated due to the frictional work is absorbed by the friction plates as well as via convection due to oil flow through the clutch plates. Excessive heat generation, however, can degrade the lubricating fluid as well as damage the clutch plates.
Additionally, traction control systems require the clutch actuator system to respond to torque commands in a quick and accurate manner. The ability to accurately meet the torque request is largely dependent on the coefficient of friction of the clutch plates. However, it has been demonstrated that this coefficient can change quite rapidly under various loading and/or slip conditions. Specifically, the coefficient tends to fade due to significant temperature increases in the clutch plates which results from insufficient heat removal. It has, however, been demonstrated that improvements in the flow of oil to the friction clutch can improve the stability of the friction coefficient. Specifically, the lube flow rate across the friction clutch has a significant impact on stability of the friction coefficient, particularly during continuous slip conditions. Furthermore, it has been demonstrated that coefficient stability can be maintained over a given time period at various engagement cycles of the friction clutch by varying the lube flow rate. As is known, the heat removal rate is dependent upon lubricating fluid flow rate and condition of the lubricating fluid.
Traditional lubricating/cooling systems include a shaft-driven pump that delivers lubricating fluid to the clutch pack. The shaft-driven pump is typically a unidirectional pump that provides no lubricating fluid flow when the vehicle is operating in a reverse mode, even though torque requests may still occur. For instance, the vehicle may be subjected to backing up on dirt, gravel or a snow-packed hill when torque transfer during four-wheel/all-wheel drive operation is needed. Additionally, the shaft-driven pump is always driven whenever the vehicle is in forward motion. In many cases, however, lubricating fluid is not required until heat is actually generated on the friction clutch components during, for instances, clutch plate slip conditions. Because the shaft-driven pump is always pumping, inefficiencies are realized and fuel economy can be negatively impacted.
Further, most high thermal loading events of the friction clutch occur at lower vehicle speeds. Therefore, the pump capacity of traditional lubricating systems is typically increased for the sake of being able to deliver more lubricating fluid to the friction clutch at low shaft speeds. Increasing pump capacity may further increase the negative impact on fuel economy, as well as creating potential for pump cavitation at higher shaft speeds. Thus, the need exists to develop improved lubrication/cooling systems for use in power transfer devices which overcome the shortcomings of conventional shaft-driven lubrication pumps.